1. Field of the Invention
The present invention relates to a copier, printer or similar electrophotographic image forming apparatus and more particularly to a color copier, a color printer or similar color image forming apparatus capable of preventing colors constituting a color image from being shifted from each other.
2. Description of the Background Art
Today, color copiers, color printers and other color image forming apparatus are extensively used and include an electrophotographic color image forming apparatus of the type using a semiconductor laser as a light source. In this type of color image forming apparatus, a laser beam modulated in accordance with a color image signal output by reading a document scans a photoconductive element, photoconductive belt or similar image carrier, thereby forming an electrostatic latent image on the image carrier. The latent image is then developed by toner of corresponding color to become a toner image. Subsequently, the toner image is transferred to a sheet.
Another type of color image forming apparatus includes a plurality of image forming units each being assigned to a particular color. Generally, this type of color image forming apparatus includes four image forming units respectively assigned to black, yellow, magenta and cyan and is, in this sense, called a four-tandem engine. Images of four different colors are sequentially transferred to a single sheet or recording medium one above the other to thereby complete a full-color image. A problem with this type of image forming apparatus is the shift of the different color components or colors from each other, i.e., color shift.
More specifically, in a digital color copier, for example, the color shift is ascribable to an error in the mounting position or the peripheral speed of each photoconductive element, an error in an exposing position assigned to each photoconductive element, an error in the linear velocity of an image transfer belt or similar endless conveying means and so forth. It has been customary to reduce the color shift by increasing the dimensional accuracy and mounting accuracy of the individual structural part. This, however, increases the cost of parts and assembling cost and thereby lowers productivity. Another problem is that readjustments are necessary due to the aging or the replacement of the individual structural part.
To solve the problems stated above, Japanese Patent Publication No. 7-19083 and Japanese Patent No. 2765626, for example, each propose an image forming apparatus configured to form exclusive color-by-color pattern images for measurement. The proposed apparatus measures, by sensing the pattern images, errors in the image forming timings of the individual colors and corrects a write start timing color by is color on the basis of such errors.
However, the image forming apparatus with the above configuration uses polygonal mirrors or similar deflectors for color-by-color writing. It is therefore necessary to control the rotation phase of each deflector before writing an image of corresponding color and to accurately drive the deflector at a constant speed during image formation. Consequently, when the color shift is corrected color by color only by the write start timing, the correction timing is limited to an integral multiple of a period of time necessary for a single scanning. Correction with accuracy higher than the period of time necessary for a single scanning is not achievable without adjusting the rotation phase of the deflector. However, the rotation phase of the deflector cannot be adjusted unless the deflector rotation speed is adjusted by temporary acceleration or deceleration. In practice, such phase correction is not only time-consuming but also technically difficult to practice and costly because the deflector rotates at high speed.
Although the above documents do not describe correction with accuracy higher than the period of time necessary for a single scanning specifically, they fail to effect such correction for the reasons described above and cannot obviate an error ascribable to short accuracy.
Further, in an image forming apparatus of the type deflecting laser beams assigned to different color images with a single deflector in order to reduce cost, the writing phases of different colors are always identical and therefore cannot be adjusted in phase color by color. It follows that when the color shift is corrected color by color only on the basis of the write start timing, correction must be done by using only the unit period of time necessary for a single scanning.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 10-157207 and 2001-13432.
It is an object of the present invention to provide a color image forming apparatus capable of accurately correcting color shift color by color when writing each color image with a laser beam to thereby enhance image quality and reliability.
A color image forming apparatus of the present invention forms a composite color image by superposing a plurality of images of different colors on a recording medium. The apparatus includes a deflector for deflecting a laser beam, a writing device for writing a latent image on a photoconductive element with the laser beam deflected by the deflector, a plurality of developing means each for developing the latent image assigned to a particular color to thereby form a corresponding toner image, and a conveyor for conveying the recording medium. A drive circuit causes the writing device to shift the optical axis of the laser beam. A computing circuit determines the amount of positional shift of each image of a particular color. A first correcting circuit corrects, based on the positional shift determined by the computing circuit, a timing at which the writing device starts writing each image of a particular color. A second correcting circuit causes, based on the positional shift determined by the computing circuit, the drive circuit to shift the optical axis of the laser beam to thereby shift in the subscanning direction the position of the photoconductive element where the laser beam is incident.